1. The Field of the Invention
The invention relates to a solar absorber with an absorber body that has one side oriented toward a concentrator and one side oriented away from concentrator and is equipped with a selective absorption layer, which, below a threshold wavelength, absorbs the spectrum of solar radiation and above the threshold wavelength, suppresses the reradiating capacity of the absorber body. The invention also relates to a solar absorber having an absorber tube and a casing and a side oriented toward a concentrator and a side oriented away from the concentrator.
2. The Related Art
The solar absorber is the part of a solar collector in which the solar radiation is converted into heat. In addition to optical losses, thermal losses also occur in the absorber since it assumes a higher temperature than the environment due to the conversion of the solar radiation into heat. In concentrating collectors such as parabolic trough collectors, Fresnel collectors, and solar power towers, the absorber is typically heated to several hundred degrees Celsius. The reradiation losses, which are proportional to the temperature raised to the fourth power according to the Planck's radiation formula, predominate over convective losses in vacuum-insulated absorbers.
The technical problem lies in the contradictory desire that on the one hand, the absorber should absorb the solar radiation well and on the other hand, should have a low emission of the characteristic radiation. According to Kirchhoff's law, the spectral obstruction and reradiating capacity of a body are equal. This problem can be solved by means of an optically selective coating that has a threshold wavelength below which the absorption capacity is high and above which the emitted characteristic radiation is low. Such an absorption layer makes use of the fact that the radiation to be absorbed and the emitted characteristic radiation of the absorber to be suppressed have different wavelength ranges.
In WO 97/26488, absorber bodies are described that have an optically selective coating. These coatings are characterized in that on the one hand, they have a good absorption capacity for the shortwave radiation (maximum at 0.5 μm) radiated by the sun and on the other hand, the absorber has a low reradiation of the longwave radiation (maximum at 3.5 to 5 μm) to the environment. The selective absorption layers have a low spectral reflectivity and high absorbitivity in the shortwave solar spectral range, while in the longwave spectral range, they have the highest possible spectral reflectivity, which is synonymous with a low emissivity.
The ideal threshold wavelength at which the transition from low to high reflectivity occurs primarily depends on the operating temperature of the absorption layer. For this reason, different selective layers have been developed for flat-plate collectors, vacuum tube collectors, and parabolic trough collectors.
Up to now, only the temperature of the absorber has been taken into account in designing the optimal threshold wavelength of the ideal optically selective absorption layer. Depending on the temperature, the threshold wavelength is adapted to a standard solar spectrum. The ideal threshold wavelength, however, also depends on the concentration of the solar radiation. In concentrating solar collectors, the concentration factor is frequently not distributed homogeneously over the absorber surface. Thus in a parabolic trough collector, only approximately half of the absorber tube is struck by concentrated solar radiation while the other half is illuminated by unconcentrated solar radiation. On the side oriented toward the concentrator, the radiation is typically concentrated by factors of 10 to 100 times. In an absorption layer that is homogeneously selective over the circumference of the absorber body, the thermal losses due to reradiation are therefore not minimized.